Cliff-Dwelling Bees Carve Their Nests Out of Solid Rock

Bees built their homes in cliffs long before the people of the American southwest did, according to a new report in Current Biology magazine. The bees not only mine hard rock with their bare mandibles, but in an amazing coincidence, they have been found to do so even within the walls of old pueblos – a cliff dwelling within a cliff dwelling.

The bee is named Anthophora pueblo, and it uses a combination of water and its own strong mandibles to carve nests out of sandstone.

Cliff-dwelling bees, by contrast, simply seem to have adopted the life of desert hermits in their painstakingly excavated cells. There do appear to be advantages to life in a mineshaft, however, and given the prevalence of the behavior, they must pay off.

A. pueblo was first discovered 40 years ago in central Utah’s San Rafael Desert. Somehow, the specimens ended up in dusty museum cabinets or drawers until 2015, when a team of Utah researchers (including the original discoverer) started looking for more of these bees in the wild. They discovered five new nest sites in the process, including one within ancient pueblo cliff dwellings.

Extraordinary though its abilities are, the bee does have limits. Their chitin mandibles are not diamond-tipped drill bits. When the sandstone becomes too hard, the bee will excavate silt or mud instead. The softer rocks preferred by the bees contain less quartz and more clay than the sandstones they shun.

Carving nests out of solid rock does have downsides. It takes more time and energy than tunneling through sand or soil, and, in spite of choosing softer rock, the mandibles of older female bees are visibly worn compared to non-sandstone dwelling bees, limiting their lifespan. Tunneling through rock also seems to make bee nests denser, which increases undesirable “nest site competition”. High-density living can also attract parasites, as anyone who has lived in a dorm can attest.

There are upsides, though. Hard rock tunnels obviously last longer than tunnels made of dirt or wood and may serve several generations of bees. Bees that nest in sandstone may also opt to delay their emergence in very dry, sparsely flowered years, though how the bees gauge outside dryness from inside a sandstone tunnel is hard for me to fathom. At least some bees delayed emergence for four years before finally judging the time right.

Rock tunnels are obviously more likely to withstand the environmental rigors of four years’ worth of flash floods and erosion than other materials. Indeed, the lifespan of these dwellings can far exceed that. The site originally discovered 40 years ago still exists, and represents the longest-lasting wild solitary bee nesting aggregation known.

Sandstone may also reduce the pathogen and parasite load; in austere rock tunnels, there is simply less to eat than in organic soil or wood, which necessarily reduces the number of potential pathogens hanging around. Rock walls also make it harder for marauding meloid beetle larvae to break into a new cell from a freshly pillaged one. On the other hand, some co-nesting parasites also delayed their emergence to synchronize with Anthophora’s emergence; presumably they too benefited from the stony homes without having to dig the tunnels themselves.

That bees can drill homes into stone seems amazing, and the fact they even build their homes in old pueblos doubly so. One wonders if ancient human cliff dwellers were ever troubled by these insects. Are sandstone-mining bees the pueblo equivalent of the termite?

ABOUT THE AUTHOR(S)

Jennifer Frazer

Jennifer Frazer is a AAAS Science Journalism Award-winning science writer. She has degrees in biology, plant pathology/mycology, and science writing, and has spent many happy hours studying life in situ.

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